1. Technical Field
This invention generally relates to feedback in audio systems. More particularly, this invention relates to identifying a frequency of feedback and adaptive filtering of feedback signals in an audio system.
2. Related Art
An audio system typically includes an input transducer (microphone), an amplifier, a microprocessor and an audio output (loudspeaker). The input transducer receives sound into the system, the amplifier amplifies the sound, the microprocessor performs signal processing, and the audio output (loudspeaker) provides sound to users of the system. Many audio systems allow for a duplex operation, where sound may be input to the microphone while audio is provided at the speaker. However, when the microphone receives a portion of the audio provided at the speaker as an input, an unstable, closed-loop system is created, resulting in audio feedback.
Audio feedback is manifested as one or more audio feedback signals at the speaker, where each feedback signal may be modeled as a sinusoidal signal (i.e. the feedback signal(s) exhibit characteristics of a sinusoidal signal). To eliminate a particular feedback signal, the microprocessor converts the audio signal into a discrete (sampled) frequency spectrum representation, such as a Discrete Fourier Transform (DFT), Spectral Estimation, Filter Banks, or like representation. The conversion of the audio signal to the sampled frequency spectrum allows for a general identification of the frequency of the feedback signal. The frequency sample having the greatest magnitude in the discrete frequency domain is selected as the frequency of the feedback signal.
A notch filter is placed at the identified frequency of the feedback signal to eliminate that particular feedback signal. However, because of computational and memory limitations of the microprocessor, the sampling resolution of the sampled frequency spectrum representation is limited. Thus, the selected frequency sample does not provide an accurate estimate of an actual frequency of the feedback signal. Because the selected frequency sample is not an accurate estimate, a notch filter is utilized that has a significantly wider bandwidth and/or a greater cut-depth than what is actually necessary for filtering the feedback signal. The wider bandwidth and/or greater cut-depth are necessary to ensure that the feedback signal is eliminated from the output signal. However, the use of a wider bandwidth and/or greater cut-depth notch filter can degrade the audio quality of the sound at the speaker.
The computational and memory limitations of the microprocessor limits the number of notch filters that may be used to eliminate audio feedback signals. Where the number of feedback signals exceeds the number of notch filters available, some of the feedback signals cannot be eliminated by the system. The failure to eliminate at least some of the feedback signals may require a system gain to be reduced, resulting in degraded system performance.